Difficulty falling asleep or remaining asleep is a significant medical issue that arises for a variety of reasons. Sometimes, these problems arise from endogenous conditions such as sleep apnea or insomnia. Other times, these problems arise from exogenous stresses such as the disruptive effect of shift work schedules and “jet lag.” Whether caused by an endogenous or exogenous source, difficulty falling asleep or remaining asleep can result in problem sleepiness, which impairs the health, quality of life and safety of those affected.
Existing pharmaceutical treatments for inducing sleep include sedatives or hypnotics such as benzodiazepine and barbiturate derivatives. These treatments have numerous drawbacks, including rebound insomnia, delayed onset of desired sedative effects, persistence of sedative effects after the desired sleep period, and side effects due to nonspecific activity such as psychomotor and memory deficits, myorelaxation, and disturbed sleep architecture, including REM sleep inhibition. Additionally, sedatives and hypnotics can be habit forming, can lose their effectiveness after extended use and may be metabolized more slowly by some people.
Consequently, physicians often recommend or prescribe antihistamines as a milder treatment for sleep disorders when hypnotics are less appropriate. However, many antihistamines still have a number of side effects. Other issues include prolongation of the QT interval in a subject's electrocardiogram, as well as central nervous system (CNS) side effects such as decreased muscle tone, drooping eyelids and drowsiness. Finally, such compounds can bind to muscarinic receptors, which leads to anti-cholinergic side effects such as blurred vision, dry mouth, constipation, urinary problems, dizziness and anxiety.
As a result, there is a need for sleep-related treatments that are antihistamines with reduced side effects. Additionally, while known sleep-inducing compounds are effective for treating sleep-onset insomnia, i.e., a subject's difficulty in falling asleep, there are no drugs currently indicated for treating sleep maintenance insomnia, i.e., maintaining a subject's sleep throughout a normal sleep period after falling asleep. Therefore, there is also a need for improved pharmaceutical treatments for maintaining sleep in subjects in need of such treatment.